Add --tag flag to agent for custom metric metadata

[stympy]

Summary

• Adds a repeatable --tag key=value flag (short: -t) to hb agent so users can annotate metric events with custom metadata like environment, role, or a custom hostname
• Tags can also be set via YAML config under agent.tags
• CLI flags take precedence over config file tags
• --tag host=foo overrides the auto-detected hostname; all other built-in metric fields (ts, event_type, used_percent, etc.) are reserved and rejected at startup with a clear error
• Updates README with config and CLI examples

Test plan

• Unit tests for tag parsing, merging, config loading, and reserved key validation
• Integration tests verifying tags appear in all event types (CPU, memory, disk)
• Integration test confirming host tag overrides auto-detected hostname
• Verify --tag event_type=foo is rejected at startup

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds support for custom user-defined tags on hb agent metric events via a repeatable --tag/-t key=value flag and agent.tags config, with CLI tags overriding config.

Changes:

• Added --tag/-t support to hb agent, with parsing, reserved-key validation, and config/CLI merging.
• Updated metric submission so tags are injected into the JSON payload (including host override behavior).
• Added unit/integration-style tests for tag parsing/merging and tag presence across submitted events; updated README examples.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
cmd/agent.go	Implements tag flag/config loading, reserved key validation, and tag merging into metric payload JSON.
cmd/agent_test.go	Adds coverage for tag parsing/merging, config loading, and verifying tags/host override appear in emitted events.
README.md	Documents agent.tags config and --tag CLI usage examples, including hostname override.

Comments suppressed due to low confidence (1)

cmd/agent_test.go:242

• reportMetrics is synchronous; the time.Sleep(2 * time.Second) here doesn’t cause additional events to be emitted and only slows the test. Consider removing the sleep and asserting on receivedEvents immediately after reportMetrics returns.

		err := reportMetrics("test-host", nil)
		require.NoError(t, err)

		// Wait for metrics to be reported
		time.Sleep(2 * time.Second)

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